1. Field of the Invention
The present invention relates generally to a semiconductor device including a ferroelectric capacitor having a thin-film structure and a method of manufacturing the same.
2. Related Background Art
There is known a non-volatile semiconductor memory device using a ferroelectric capacitor (which will be hereinafter referred to as a “ferroelectric memory”). The ferroelectric capacitor is formed by stacking a bottom electrode, a ferroelectric film and a top electrode on a substrate. A typical ferroelectric film is formed of a perovskite compound, such as lead zirconate titanate (PZT; PbZrxTi1−xO.3) (0<x<1), which has a perovskite type crystal structure. When a PZT film is used, a platinum (Pt) film is typically used as each of the bottom and top electrodes. The ferroelectric memory is capable of non-volatile storing data by the spontaneous polarization (remnant polarization) of the ferroelectric.
Such a ferroelectric memory is capable of holding data without the need of any batteries and of operating at a high speed, so that it begins being applied to a non-contact card (RF-ID; Radio Frequency-Indication) and so forth. The ferroelectric memory is also exactly expected to be substituted for the existing SRAM, EEPROM flash memory, DRAM or the like, and to be applied to a logic mixed memory.
However, in the conventional ferroelectric capacitor using PZT, hydrogen causes the decreases of the amount of spontaneous polarization, so that the signal margin in FRAM is decreased resulting in the bit failure of the memory, and the decrease of reliability and yield of the whole memory.
It has been reported that the spontaneous polarization of a ferroelectric capacitor having a Pt/PZT/Pt structure is degraded by annealing in an atmosphere of H2 (e.g., J. Appl. Phys. Vol. 82, No. 1, July 1997, pp. 341-344). On the other hand, although a technique for improving the characteristics of a ferroelectric capacitor by decreasing the concentration of hydrogen in the ferroelectric capacitor is disclosed in, e.g., Japanese Patent Laid-Open No. 8-8404(1996), this technique is not realistic since there is a strict limit to a process.
In addition, a basic process for manufacturing the above described ferroelectric capacitor having the Pt/PZT/Pt structure comprises the steps of: sequentially depositing a bottom Pt electrode and a PZT film on a substrate; carrying out a heat treatment for crystallizing the deposited PZT film; and forming a top Pt electrode on the PZT film.
It has been also reported that the spontaneous polarization of the ferroelectric capacitor having the Pt/PZT/Pt structure is degraded by a heat treatment in an atmosphere of hydrogen. This is caused by the fact that a large amount of oxygen vacancy is introduced into the PZT film by the reducing power of hydrogen and the catalysis of the Pt electrode.
On the other hand, it has been revealed by the inventor's recent study that when the PZT film is deposited on the bottom Pt electrode to be crystallized, Pt is diffused into PZT film from the bottom Pt electrode in deep range of the PZT film to form a conductive layer. Because the state of PZT film is an amorphous state so that the rate of the diffusion reaction thereof is high when the PZT film is deposited.
In particular, if the thickness of the PZT film is intended to be decreased to 100 nm or less, most of the PZT film becomes the conductive layer. Therefore, it is difficult to carry out the scaling of the thickness of the ferroelectric film.
FIG. 1 shows an example of the processed shape of a ferroelectric capacitor on a substrate 71. In order to achieve the scale down and high integration of a memory, a bottom Pt electrode 72, a PZT film 73 and a top Pt electrode 74 are preferably sequentially etched in substantially vertical directions to form a ferroelectric capacitor as shown in FIG. 1. However, if it is intended to obtain such a shape of capacitor, there is a problem in that Pt splashed by etching the bottom Pt electrode 72 is adhered to the side wall of the PZT film 73 again to form a Pt fence to establish a short-circuit between the top and bottom electrodes.
Moreover, as shown in FIG. 2, the ferroelectric capacitor is finally covered with a passivation film 75, and connected to an interconnection 76. In this structure, the end portion of the layer between the PZT film 73 and the bottom Pt electrode 72 directly contacts the passivation film 75. Therefore, when hydrogen annealing is carried out, hydrogen passing through the passivation film 75 penetrates into the layer between the PZT film 73 and the bottom Pt electrode 72 to cause degradation, such as the decrease of the amount of polarization and the peeling of films, with age.
It has been known that the characteristics of the ferroelectric capacitor are degraded by hydrogen during a process for manufacturing an Si-LSL, specifically that the amount of polarization thereof is decreased by hydrogen during the process. In addition, it has been proposed that a TiO2 film, an Al2O3 film or the like can be effectively used as a protective film for protecting the ferroelectric capacitor against such penetrating hydrogen (e.g., see IEDM 97-609-612, IEDM 97-617-620). However, in the shape that the layer between the PZT film 73 and the bottom Pt electrode 72 is continued from the surface of the bottom Pt electrode 72 extending to the outside of the PZT film 73 as shown in FIG. 2, even if the hydrogen protecting film is provided, it is difficult to interrupt the reducing element, such as hydrogen, which penetrates into the layer between the PZT film 73 and the bottom Pt electrode 72 along the surface of the bottom Pt electrode 72, so that it is not possible to surely prevent the degradation in characteristics.